Earth Engine workshop exercises

Can we simplify this script?

Exercise: Instead of returning a FeatureCollection of sampledPoints around each buffered point, modify the samplePoints function so that it:

1. Takes the mean of all the samplePoints within a bufferzone using the same “reduceColumns” method I showed you earlier.

2. Creates a feature with a null geometry and the bufferzone mean assigned to the buffered point ID from which the points were sampled.

3. Uses this new function and a mapping operation to create the meanPopDensFC variable without needing to compute sampledPoints, meanPopDens, or meanPopDensList.

time:30 minutes

Modified samplePoints function

// Function to sample random points within each buffered area
var samplePoints = function (feature) {
  var geometry = feature.geometry();
  var randomPoints = ee.FeatureCollection.randomPoints(geometry, 10);
  var sampledPoints = popDensYearBuffered.sampleRegions({
    collection: randomPoints,
    scale: 1000,
    tileScale: 16
  })
  // Add the buffered area ID to each sampled point
  sampledPoints = sampledPoints.map(function(point) {
    return point.set('BufferedAreaID', feature.get('ID'));
  });

  // Calculate the mean 'population_density' for each group
  var meanPopDens = sampledPoints.reduceColumns({
    reducer: ee.Reducer.mean(),
    selectors: ['population_density']
  });
  return ee.Feature(null, {
    'BufferedAreaID': feature.get('ID'),
    'mean': meanPopDens.get('mean')
  });
};

// Sample 10 random points within each buffered area and calculate the mean population density
var meanPopDensFC = bufferedPoints.map(samplePoints);

Seperating out functions

Another way to simplify might be to create a new module to store functions we have created in this script.

Exercise:

• Create an additional file called “Helpers”.
• Then use the “Docs” tab in the lefthand pane to research how to include this file’s exported functions into your port finding script.

time: 10 minutes

How to include your helper module

To include this module we will add the following line to the beginning of our port finding script:

// Import the lineToPoint function
var hlp = require('users/dylanblee/REUworkshop:Helpers');

Put the lineToPoint function into your helper module

Exercise:

• Research how to export a function from a javascript script.
• Migrate the “lineToPoint” function from port finder script to helper module.

time: 20 minutes